Designer Benefits
Structural steel designers are not in the business of taking chances with their creations. They know that some structures can be assembled with loose bolts, "snug tight" bolts or A307 bolts, and no distress would ensue. But they also know that some structures need correctly pretensioned bolts to keep the faying surfaces in firm contact and to resist the design service and ultimate loads.
DTI's fulfill the requirements of the "special inspection" provisions of ICBO, which are often required to be satisfied on particular connections which ICBO stipulates must be correctly assembled and checked.
Just about all connections on structural steel are governed by the Research Council on Structural Connections. According to the Research Council on Structural Connections (RCSC), the "contract documents" (read "design drawings") must show the "Joint Type" - either snug-tight, pretensioned, or slip-critical.
Designers know that the following types of connections require full pretension in the bolts:
- All bridges.
- All slip-critical connections. (The Research Council on Structural Connections states the connection must be designed as "slip-critical" where slip would be detrimental to the behavior of the joint, where the joint will experience fatigue or significant load reversal, joints with oversized or slotted holes, where welds and bolts are used together, and where the bolts are used in direct axial tension) (Revised in the 2000 RCSC to say you really don't have to tension all bolts used in pure tension if statically loaded) .
- Where the bolts are considered to be in the category of "pre-tensioned shear/bearing", they must also be fully pretensioned, although the degree of inspection for bolt tension is not mandated. (In the 2000 RCSC Spec inspection is mandated)
- Where the faying surfaces are coated and the design calls for transfer of loads by friction, the joint surface must be qualified and the bolts must be fully pretensioned also.
- Most engineers require correctly tensioned bolts in connections supporting moving machinery, connections governed by seismic loads, connections between the members of a wind resisting system, moment connections, connections in steel which shares load with concrete members, in roof truss splices, in roof truss bracing and connections of roof trusses to their columns, where the prying load on arrays of bolts is high or indeterminate, where the fit between members is or is suspected to be poor (read pre-engineered end plate connections), and in connections of frames having a high slenderness ratio. The AISC LRFD Spec now states that built-up compression members must also have fully pretensioned bolts so that their slenderness ratio is predictable.
The design engineer has to decide whether DTI's should be specified as the ONLY bolt installation quality assurance method on the fully pretensioned bolts, just as decisions on paint quality or steel grade or welding procedure must be made. Often the decision is yes. Leaving the decision of whether or not to use DTI's up to the other members of the construction team will often result in shortcuts being taken in bolting. Designers are not paid to take chances that something MIGHT be constructed correctly, and specifying DTI's sends a message to the trades that the design is important. Designers usually acknowledge that DTI's produce higher and more consistent bolt tensions than any other method, and if that is what is needed on all or part of a structure, then they should be specified.
DTI's do not affect the overall cost of a steel frame because the cost of bolting inspection is reduced. In fact, when SQUIRTER® DTIs are used, it has been demonstrated that the bolt installation and inspection cost is reduced, by about $1.00 per bolt, by one measurement. Click here to see the trial study results
Occasionally designers must justify the cost (see Total Installed Cost) of DTI's to the project owner, because, as an added item to purchase, their cost is identifiable, although small. At 15 bolts to the ton of steelwork, DTI's add about $7.50/ton to the $1500 to $2000 per ton cost of steelwork, or less than one half of one percent. When DTI's are specified for only some of the connections in a structure, the cost of them becomes even smaller.
Torque wrench inspection and part-turn inspection is unnecessary when DTI's are used, saving $5.00 to $8.00 per ton. That's why we can say that, overall, the owner will not pay any premium (see Total Installed Cost) when DTI's are used, often less.
Designers should carefully consider the use and specification of SQUIRTER® DTIs, because they save money and improve constructability.
